Steps to reproduce:
- As admin, create a new course.
- Add a grade item with certain settings.
- Log in as 'Bad teacher', and change some of the grade items settings.
- Log in as admin, and visit the gradebook.
- The settings are now wrong, but you don't know who changed them.
- You should be able to go to Course administration -> Logs, to see who made the change, so you can discus it with them.
- The grade history report gives you some information.
All singificant changes made in Moodle should be logged, therefore, I think, the missing events here should be considered a bug.